This invention pertains to a method and coating composition useful for coating glass so as to provide lubricity and abrasion resistance to the surface of the glass. More particularly, it pertains to such a method and composition suitable for application as a bare, freshly formed glass surface without pretreatment and particularly without a prior "hot-end" coating or treatment.
Due to the nature of raw glass surfaces, abrasion occurs whenever two such surfaces come in contact with each other or in contact with equipment used for handling a glass product. Any scratches or flaws in a glass surface may cause a decrease in strength of the glass to as little as one-fourth of its original value. Generally, glass articles of commerce, such as jars, fibers, tubes, pipes, bottles, tumblers, and the like are strongest when freshly formed. This strength decreases as the glass articles come into contact with each other and with other surfaces in the course of manufacturing, packaging, filling, and shipment.
It is, therefore, desirable for a glass surface to be coated with a composition having good lubricity and scratch or abrasion resistance properties. This decreases the likelihood of breakage, permitting more bottles, for example, to be handled by high-speed filling and packaging apparatus even though the glass surfaces will be subject to more contact with each other and with other surfaces, such as in glass to glass shrink wrapped bulk palletizing.
In the past, numerous types of such protective coatings have been developed. The compositions of such coatings include polyethylene waxes, acrylic-ethylene copolymers, complex stearates, fatty acids, and its derivatives, polyurethanes, vinyl copolymers, and silicones. Such coatings are generally applied after the glass is annealed and close to room temperature; therefore, these coatings are called "cold-end" coatings. One feature of all of these known "cold-end" coatings is that they are only effective in providing abrasion protection on glass surfaces that have been pretreated with a "hot-end" coating. Typically, hot-end coatings are formed by depositing a pyrolyzable titanium or tin compound, which chemically decomposes upon heating to form corresponding oxides of the metal, onto the glass surface while the surface is at a temperture above the pyrolyzing temperature of the compound (usually between 700.degree.-1300.degree. F.).
Such hot end coatings are applied soon after the article leaves the glass-forming machine and before it is cooled in the annealing lehr, after which one of the above-mentioned prior art types of "cold-end" coatings is applied. The "hot-end" surface treatment does not offer increased protection or lubricity when used by itself but merely alters the composition of the bonding surface and allows the prior art "cold-end" coatings to form a uniform durable film. Such pairs of "hot-end" and "cold-end" coatings have been in use for many years in the glass container industry. However, the use of "hot-end" surface treatment is an added expense and requires expensive exhaust systems with special stacks and scrubbers to remove the toxic and corrosive vapors produced from the decomposition products of "hot-end" coating compositions. In addition, these "hot-end" surface treatments have to be applied to bottles and containers in a manner such that the "finish" (i.e., the neck) of the bottle is untreated to avoid screw cap removal problems. Another problem commonly encountered in industry is an undesirable silvery appearance on "hot-end" treated ware due to an excess of the metal oxide deposit on the surface.
Certain of the prior art glass treating compositions have specific objectionable characteristics. For example, polyethylene waxes and silicones form a continuous hydrophobic surface on the glassware which is not receptive to conventional label adhesives and decorative inks and hence have to be removed prior to labelling, printing, or decorating. While the stearate soap type coatings are satisfactory from a lubrication and protection standpoint when immediately applied over "hot-end" treated bottles, such coatings are soluble in water and are removed when the ware is subjected to long term storage in humid atmospheres or prior to filling, autoclaving for sterilization, or retort processing or pasteurizing of filled ware.
It is apparent, therefore, that there is a significant need for a commercially practical method and composition by which a glass surface can be treated with a "cold-end" coating composition (i.e. without prior "hot-end" treatment), which provides desirable properties of durability, lubricity, clarity, and abrasion protection to glass and glassware. To the extent specific "cold-end" coatings have been proposed, significant problems, such as lack of clarity, are encountered with all such known coatings.
For example, oleic acid "cold-end" coating, deposited in vapor form may contaminate the glassware interior. (Internal contamination results when, during the application of the coating to the exterior surface of the glassware, a small amount of the coating enters the mouth of the glassware. Where the possibility of such contamination exists, it is essential that the coating ingredients be selected from materials in compliance with FDA regulations.) Poor label adhesion may also be experienced when an excess of the oleic acid is deposited on the glassware surface.
A mixture of polyvinylalcohol and polyoxyethylene stearate has apparently been used (proposed in U.S. Pat. No. 3,712,829) for coating ware without prior hot end coating. But this coating is suitable only where high abrasion resistance and permanent coating is not required.
A carboxyl functional polyamino acrylate ester resin crosslinked with a water soluble crosslinking agent such as epoxy or formaldehyde condensation resins and blended with carnuba wax and a silane coupling agent is claimed to provide abrasion protection to glassware with no hot-end pretreatment (in U.S. Pat. No. 4,224,365). However, this type of treatment requires a heavy coating thickness of about 10 to 25 microns and abrasion protection is minimal. Also the coating composition requires high temperature cure for several minutes which is not practical in many high speed glassware production lines.
In view of these problems, it is the object of this invention to provide a method and composition for applying an aqueous coating solution to glassware, which requires no prior "hot-end" pretreatment, and which produces a coating having desirable properties, at very low coating thickness, including clarity, lubricity, abrasion protection, permanency, resistance to hot water washing, autoclavibility, FDA compliance, and receptiveness to conventional label adhesives and decorating or printing inks.
It is also an object of this invention to provide a method and composition for applying an aqueous coating solution to glassware, with or without hot-end pre-treatment which is useful on glassware adapted to be sterilized by dry-heat techniques (elevated temperatures without the presence of high humidity) at temperatures of up to 400.degree. F. for 4 hours with no discoloration.
Another object of this invention is to provide an aqueous coating composition that can also provide abrasion protection to glassware subjected to an automatic dishwasher detergent wash.
A further object of this invention is to provide an aqueous coating composition that can be applied to glassware by conventional coating application methods such as spraying, roller or brush coating, and dipping.